Targeted vaccination of people potentially
exposed to smallpox during a bioterrorist episode,
together with an appropriately low level of vaccination
before an attack, provides the best protection
against smallpox, according to an analysis by U-M
Prof. Jim Koopman in the Nov. 15 issue of the
journal Science.

But questions remain because of the lack of
relevant data and because there is scientific
modeling work left to be done, Koopman, professor of
epidemiology at the School of Public Health, writes in
a Perspectives article in the journal.

In the absence of ongoing cases of smallpox
to study, Koopman writes that scientists must use
the best possible mathematical models to
hypothesize what might happen if bioterrorists were to
unleash the highly contagious disease, which the CIA
suspects is held by four unauthorized countries
including Iraq and North Korea.

Assessing the pros and cons of mass
vaccinationsthe administering of the vaccine to
anyone who comes in to get itversus targeted
vaccinations by health department personnel going to those
who must be vaccinated in order to stop spread, Koopman evaluates the infection transmission
models used in two recent studies of how smallpox
might move from person to person.

A continuous population model treats people as being blended together; if
any part of the population is infected, transmission to the susceptible
part of the population will occur. A discrete individual model instead
looks at who is infected, and how and to whom they might pass along the
disease.

Koopman favors the discrete individual approach because its computations
take into account chance events that often end chains of disease transmission.
Because most chains of smallpox die out locally, continuous population
models don't reflect reality; they assume that as long as infectious and
susceptible people co-exist, they will come into contact and the disease
will continue to spread.

Regardless of which approach they use,
scientists build their models on limited data, Koopman writes.
He suggests crafting smallpox models by analogy
to influenza outbreaks, and collecting more data
to support more realistic modeling. Specifically he
suggests analyzing influenza virus DNA from
infected individuals with known exposures to
understand how influenza flows through a population.

The debate is not simply one of logistics and where scarce resources should
be deployed first. Estimates from the 1960s, when the smallpox vaccine
was last mass administered, show that about 15 out of every million people
will experience life-threatening side effects, and one or two are likely
to die from the effects of the vaccine. Meanwhile, those who have been
immunized recently carry a live virus called vaccinia and can infect vulnerable
unvaccinated people around them.